I could go either way on the abilities, I guess.
Yeah, a 60 ft mental detection is along the lines of what I'm thinking, too.
I actually think the fence doesn't alert the brown unicorn, just intruders/visitors. At least, that's how it reads to me.
So any creature with a power point reserve or other brown unicorns can hear it? How do we want to specify psionic creatures since we usually stick psionics in a sidebar?
Well, that's a lot of text, but there's some potential, so I don't think I want to drop it entirely.
I'd simplify it further...
Psychic Warning Fence (Psi): A brown unicorn can use its horn to trace an invisible barrier of mental energies around an area of up to x square feet (or maybe 1 mile?). The barrier warns any creatures with a psi-like ability, power point reserve, or telepathy that they are entering the brown unicorn's territory. A psychic warning fence lasts for Y days, and can be dispelled as if it were a psi-like ability (manifester level equals unicorn's HD). A brown unicorn may only have one psychic warning fence active at a time; creating a second immediately removes the first.
Brown unicorns communicate with each other mentally. They have magical (nonpsionic) telepathy (applicable only to all other brown unicorns within range) and ESP (which reaches all other beings, one person per round) out to a 60' radius; both powers operate continuously.
They can cast a special sleep spell three times per day; this spell is exactly like the wizard spell but it affects at least one creature regardless of level. The spell works normally against creatures of 4+4 HD or less, but creatures of higher levels are allowed a saving throw vs. spells to avoid being put to sleep.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.